The Ultimate Guide to Stainless Steel Surface Finishes
The surface finish of stainless steel affects much more than just its aesthetic — performance, reliability, and other factors can be greatly impacted.
No matter the application or end-use of the stainless steel, its surface finish should be carefully chosen to best meet the requirements at hand. That's because, in addition to providing the desired visual appeal, the choice of finish affects corrosion resistance, characteristics such as electrical properties, impacts weldability, manufacturing operations, and contributes to various economic considerations.
Some applications, such as superconductor components or flexible metal hoses, demand stainless steel with a specific finish for the end-use product to perform as designed.
It's also important to note that not all suppliers use the same terminology to refer to the same finish. Keep reading to learn the subtle differences between each finish to understand better how suppliers may differ.
What Is a Stainless-Steel Surface Finish?
Surface finish is the visual appearance of the metal and is created through various processing methods, including rolling, polishing, and blasting. These finishes range from dull to bright and include special textures that may be mechanically applied for highly specialized applications.
Several factors can affect surface finish, including the production route, material thickness, the surface finish of the rolls, cleanliness, and mechanical abrading.
Most melt mills can produce all sheet and strip finishes. While some finishes (BA and polished) require specialized lines, most mills have these or a third-party processing partner. However, melt mills won't always be able to meet each customer's unique Ra (surface roughness) requirements, and a precision re-roller may be needed.
What Are the Different Types of Stainless-Steel Surface Finishes?
Surface finishes are commonly identified by standard industry designations based on process routing (not appearance) and are different for sheet (greater than 24” wide) and strip (less than 24” wide). However, as previously mentioned, some suppliers use proprietary designations or different industry standards, so it’s best to double-check that you’re on the same page.
Ulbrich's surface finish designations are based on ASTM A480 specifications. Below is an overview of the types of finishes available on our sheet products.
Mill Finishes
Whether hot or cold rolled, these finishes are the basic supply condition for all stainless-steel sheet products. They are universally used for many applications and serve as the starting point for mechanical polishing and other finishing processes. To maximize corrosion resistance, most are cleaned with acid (pickled) to remove scale that builds up during hot rolling and annealing (a bright annealed finish being the exception!)
No. 1 Finish
A No. 1 Finish is produced via hot rolling, then annealing and pickling the material in acid. This results in a dull finish and is typically used when a smooth decorative finish isn’t important, for example, in elevated temperature applications.
No. 2D Finish
A No. 2D Finish is produced via cold rolling, then annealing and pickling the material in acid. The finish is smooth, non-reflective, and dull. It is ideal for deep drawing applications and is found in auto exhaust components, hardware, and chemical equipment.
No. 2B Finish
A No. 2B Finish is cold rolled, annealed, and pickled, followed by a light temper pass using polished rolls. The finish is smooth, somewhat reflective, and bright. It’s a general-purpose cold-rolled finish used in cookware, small tanks, and pharmaceutical equipment.
Bright Annealed (BA) Finish
A Bright Annealed Finish is cold rolled, then annealed in a controlled, protective atmosphere to prevent scale formation during annealing. This finish is not pickled in acid because a thin oxide film is formed during annealing, eliminating the need for pickling. It is used in trim appliances, surgical instruments, and cookware.
Mechanically Polished Finishes
These finishes involve using abrasive materials that effectively cut the steel's surface to some degree.
No. 3 & No. 4 Finishes
No. 3 and No. 4 finishes are produced by mechanically polishing a No. 2B finish substrate. These finishes have uniform appearances that are produced by an emery cloth belt used for polishing. Each finish has a corresponding grit necessary to create its surface texture.
These finishes are among the most used in stainless steel. Applications featuring No. 3 finishes include architectural and food processing components. Applications featuring No. 4 finishes include architectural wall panels, elevators, sinks, and restaurant equipment.
No. 6 Finish
A No. 6 finish is created by Tampico brushing a No. 4 finish. It’s dull, silver-white, and less reflective than a No. 4 finish. This finish was once commonly found on stainless steel architectural components up to the 1980s but is much less used today.
No. 7 Finish
A No. 7 finish is produced by buffing a finely ground surface. The finish is high luster with grit lines still visible on the material. The result is highly reflective, almost mirror-like. Some common applications for No. 7 finishes include column covers, ornament trim, and wall panels.
No. 8 Finish
This is produced by polishing with successively higher grit abrasives and then buffing for a mirror finish. By ASTM standards, they are the most reflective of all finishes.
Because they are so reflective, No. 8 finishes have a lot of notable everyday applications, including press plates, signs, and wall panels. Most notably, Chicago’s “Bean” is stainless steel with a #8 finish.
Other Finishes
Custom Finishes
Custom metal surface finishes allow for specific characteristics to be unlocked that are not available if a standard finish is specified. Engineers for advanced and emerging applications specify the surface roughness in Ra or Rz (see below) that they need to meet their objectives.
For custom surface finishes beyond those mentioned here, contact us to connect with a metallurgical engineer who can assist.
TR (Temper Rolled) Finish
A TR finish is produced when an annealed surface is cold rolled to obtain mechanical properties. The appearance varies based on the alloy and the amount of cold work. Quarter hard, half hard, and full hard are common examples of temper-rolled finishes. This finish is used all the time and is one of the largest categories of stainless steel that we sell at Ulbrich.
Architectural Finish
An architectural finish can be achieved through special finishes at the temper mill, the final reduction stand, special pickle processes, or a combination. These are typically agreed upon between buyer and producer.
Abel Black
Abel Black™ stainless steel is made using a proprietary coloring method that preserves its metallic texture (unlike painting or plating). Electricity and chemicals are used to create an oxide film that causes light inference, making the stainless-steel surface appear black. Since the oxide film is integrated with the base metal, it will not peel off and delivers higher durability and weather resistance than untreated stainless steel.
XL Blend S
XL Blend "S" is a breakthrough stainless steel finish. It can repair surface damage, creating a clean, smooth, and uniform exterior. The finer grain structure makes it less reflective and less receptive to foreign substances, resulting in a surface that is easier to clean and maintain. Common applications include escalators, transportation, and food service appliances.
Abel Black and XL Blend S are both available from Diversified UIbrich!
What is the Surface Roughness of Stainless Steel?
Any discussion of surface finish would be incomplete without mentioning surface roughness. Surface roughness is a measure of the texture of a manufactured surface. RA or Average roughness is most commonly used in North America. It is the average of the peaks and valleys of a surface over a set sampling length. Values are typically given in microinches or micrometers.
Rz, or mean roughness depth, is most commonly used in Europe and is the average of the highest peak to the lowest valley over 5 sampling lengths.
Surface roughness can have a crucial impact on the durability and performance of the material and must also be closely considered and controlled for.
Still unsure which finish is right for your application? Contact us and speak with someone from our metallurgy or product development team. We also offer custom finishes beyond those mentioned here to accommodate advanced and emerging applications, such as superconductors.
